Method of producing ferritic iron-base alloys and ferritic heat resistant steels

ABSTRACT

A method of designing a ferritic iron-base alloy having excellent characteristics according not to the conventional trial-and-error technique but to a theoretical method, and a ferritic heat-resistant steel for use as the material of turbines and boilers usable even in an ultrasupercritical pressure power plant. Specifically, the d-electron orbital energy level (Md) and the bond order (Bo) with respect to iron (Fe) of each alloying element of a body-centered cubic iron-base alloy are determined by the Dv-Xα cluster method, and the type and quantity of each element to be added to the alloy are determined in such a manner that the average Bo value and average Md value represented respectively by the following equations: 
     
         average Bo value=ΣXi.(Bo)i                           1 
    
     
         average Md value=ΣXi.(Md)i                           2 
    
     coincide with particular values conforming to the characteristics required of the alloy; wherein Xi represents atomic fraction of an alloying element i, and (Bo)i and (Md)i represent respectively the Bo value and Md value of the element i. Preferably, the average Bo value and average Md value are, respectively, in the ranges of 1.805 to 1.817 and 0.8520 to 0.8628.

TECHNICAL FIELD

This invention relates to a method of designing ferritic iron-basealloys on the basis of a predicting system without depending uponconventional trial-and-error experimental procedures. This inventionalso relates to high strength ferritic heat resistant steels whichexhibit high temperature strength and other physical and chemicalproperties more excellent than those of the conventional ferritic heatresistant steels. The steels are particularly suitable for materials ofturbines and boilers.

BACKGROUND ART

Although heat resistant steels are used in various areas, materials ofturbines and boilers are the typical uses of the ferritic heat resistantsteels. Therefore, the heat resistant steels of this invention will bespecified in terms of turbine and boiler materials hereinafter.

Most of conventional heat resistant steels hitherto developed for use inboiler and turbine materials contained 9 to 12% chromium as well as oneor more of carbon, silicon, manganese, nickel, molybdenum, tungsten,vanadium, niobium, titanium, boron, nitrogen and copper, in amounts of0.04 to 2.0%, respectively. It should be noted that "percent (%)" means"mass %" herein unless any explanatory note is given.

Compositions of typical heat resistant steels for materials of turbinesand boilers are listed in Table 1 and Table 2 (refer to "Compositions,Structures and Creep Characteristics of Heat Resistant Alloys"distributed as a brief at the 78th conference held under co-sponsorshipof Japan Metal Society and Kyushu branch of Japan Iron and SteelInstitute . . . Reference 1). All these steels have been developed bymany experiments wherein various elements of various amounts werealloyed in turn. The action and function of each said alloying elementhas come to be known by such trial-and-error experiments and can beroughly summarized as follows.

Chromium:

Chromium improves corrosion and heat resistance of the steel. Chromiumcontent should be increased as the service temperature of the steel iselevated.

Tungsten, Molybdenum:

These elements improve high temperature strength of the steel due totheir function for bringing about solid solution hardening andprecipitation hardening in the structure of the steel. However, ascontents of these elements are increased, the ductile-brittle transitiontemperature (DBTT) of the resultant steel is elevated. In order tosuppress the embrittlement of the steel, the molybdenum equivalentMo+(1/2)W! is necessarily lowered below 1.5%. In accordance with thisinstruction, the molybdenum equivalent of most of the conventionalalloys is around 1.5%.

Vanadium, Niobium:

These elements will bring about strengthening of a steel due toformation of carbo-nitrides through precipitation hardening. The solidsolubility of vanadium in a steel is 0.2%, whereas that of niobium is0.03%, when the steel is annealed at a temperature of 1050° C. If theamount of vanadium and that of niobium exceed their respective solidsolubility, the excess amount of vanadium and that of niobium will formtheir carbides and nitrides in the steel matrix during annealing.Results of experimental work obtained up to the present, in particularthat of creep rupture tests, show that the optimum vanadium and niobiumcontents are 0.2% and 0.05%, respectively. The niobium content "0.05%"in the steel exceeds its solid solubility, and the excess niobium formsNbC which is effective to suppress coarsening of austenitic crystalgrains during annealing heat treatment.

Copper:

As copper is one of the austenite stabilizing elements, it suppressesformation of the δ-ferrite as well as precipitation of iron carbides.Copper in the steel exhibits a weak action of lowering the Ac₁ point andimproves hardenability of the steel. Copper suppresses forming asoftened layer in a heat affected zone (hereinafter designated as HAZ).However, addition of more than 1% copper to a steel decreases itsreduction of area upon creep rupture.

Carbon, Nitrogen:

These elements are effective to control structure and strength of thesteel. Concerning creep properties of the steel, the optimum carbon andnitrogen amounts for creep rupture strength depend on contents ofvanadium, niobium or the like carbide and/or nitride forming elements inthe steel.

Boron:

About 0.005% of boron in a steel improves its hardenability. It is saidthat boron is further effective to make the steel structure fine andthereby to improve strength and toughness.

Silicon, Phosphorus, Sulphur, Manganese:

In order to suppress embrittlement of the steel by making itsuper-clean, these elements are desired to be as low as possible.However, silicon has an effect of suppressing oxidizing attack of watervapor on the steel. So it is said that some amount of silicon should bekept in the boiler steel.

The action and function of each alloying element are clarified to someextent in accordance with the conventional alloy developing method, asmentioned above. However, a great deal of experimental work will berequired before obtaining a novel sort of steel with desirable chemicaland physical properties. For example, in a steel containing fivealloying elements, if the content of each element is changed in threecontent levels, 3⁵ combinations could be produced and such huge numbersof alloys have to be melted, cast and formed into various testspecimens, followed by a great deal of experimentations.

As shown in Tables 1 and 2, most of the heat resistant steels recentlydeveloped contain more than ten alloying elements. Development of newsteels like the steels in Tables 1 and 2 in accordance with theconventional trial-and-error method requires a great deal of labor, timeand cost.

We, the inventors, already developed a method of designing novelmetallic materials on the basis of a molecular orbital theory. Anoutline of the method is disclosed in "Journal of Metal Institute ofJapan, Vol.31, No.7(1992), pp 599-603" (Reference 2) and "Altopia.September 1991, pp. 23-31" (Reference 3). Meanwhile, we filed a JapanesePatent Application relating to "A Method of Producing Nickel Base Alloysand Austenitic Ferrous Alloys" refer to Japanese Patent No.1,831,647(Japanese Patent Publication No.5-40806) corresponding to U.S. Pat. No.4,824,637!.

It is certain that, in view of the above-mentioned references and patentdocuments, the novel alloy designing method is applicable to producealuminum base alloys, titanium base alloys, nickel base alloys and thelike nonferrous alloys, intermelallic compound alloys and austeniticiron-base alloys. However, it has not been certain that the novel alloydesigning system can be applicable to produce ferritic heat resistantsteels.

This invention has been accomplished to provide a novel alloy designingsystem for producing iron base alloys, particularly ferritic heatresistant steels, without the need of troublesome trial-and-errorexperimentation.

Therefore, an object of this invention is to provide a method ofproducing with high efficiency ferritic iron base alloys excellent inhigh temperature strength on the basis of theoretical predicting system.

Another object of this invention is to provide ferritic heat resistantsteels which are excellent in various physical and chemical propertiessuch as high temperature strength, as compared with the conventionalferritic heat resistant steel and therefore are well applicable toturbine and boiler materials which are durable even for a severe watervapor environment of 246-351 kgf/cm² g pressure and 538°-649° C.temperature.

DISCLOSURE OF THE INVENTION

This invention is intended to provide the following methods (1) and (2)of producing ferritic heat resistant steels, and the following ferriticheat resistant steels (3) to (5).

(1) A method of producing ferritic iron base alloys characterized inthat both d-electron orbital energy level (Md) of each alloying elementcontained in a body centered cubic iron base alloy and bond order (Bo)of each said alloying element to iron (Fe) are determined by Dv-X αcluster method, and type and amount of any alloying element to be addedto said iron base alloy are determined in such a manner that average Bovalue expressed by following formula 0 and average Md value expressed byfollowing formula 5 are kept in a respective desirable range inaccordance with the aimed chemical and physical properties of the steelto be produced.

    average Bo value=ΣXi·(Bo)i                  1

    average Md value=ΣXi·(Md)i                  2

wherein Xi is the atomic fraction of an alloying element i, and (Bo)iand (Md)i are Bo value and Md value for the alloying element i,respectively.

(2) A method of producing strong ferritic heat resistant steelsaccording to (1), wherein the above-mentioned average Bo value isrestricted in a range of 1.805 to 1.817, and the above-mentioned averageMd value is restricted in a range of 0.8520 to 0.8628.

(3) A ferritic heat resistant steel characterized in that the steelcontains, in mass % basis, 9.0-13.5% chromium, 0.02-0.14% carbon,0.5-4.3% cobalt, 0.5-2.6% tungsten, and that the above-mentioned averageBo value and the above-mentioned average Md value are located in thearea surrounded by segment AB, segment BC, segment CD and segment DA, oron one of those segments in FIG. 6.

(4) A ferritic heat resistant steel characterized by consisting of, inmass % basis, 0.07-0.14% carbon, 0.01-0.10% nitrogen, not more than0.10% silicon, 0.12-0.22% vanadium, 10.0-13.5% chromium, not more than0.45% manganese, 0.5-4.3% cobalt, 0.02-0.10% niobium, 0.02-0.8%molybdenum, 0.5-2.6% tungsten, 0-0.02% boron, 0-3.0% rhenium and thebalance iron and incidental impurities.

(5) A ferritic heat resistant steel characterized by consisting of, inmass % basis, 0.02-0.12% carbon, 0.01-0.10% nitrogen, not more than0.50% silicon, 0.15-0.25% vanadium, 9.0-13.5% chromium, not more than0.45% manganese, 0.5-4.3% cobalt, 0.02-0.10% niobium, 0.02-0.8%molybdenum, 0.5-2.6% tungsten, 0-0.02% boron, 0-3.0% rhenium and thebalance iron and incidental impurities.

The heat resistant steel (4) is particularly suitable for use as turbinematerial, whereas the steel (5) is suitable for use as boiler material.Among incidental impurities contaminating the steels (3) to (5), nickelis preferably restricted in a range of not more than 0.40 mass %.Phosphorus and sulfur are preferably restricted in a range not exceeding0.01 mass %, respectively in the steel (4).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cluster model for a calculation of Md and Bo values of abody centered cubic iron,

FIG. 2 is a diagram showing locations of average Bo values and averageMd values of alloys wherein 1 mol. % of any one of alloying elements isadded to iron, and alloying vectors of each alloying element,

FIG. 3 is a diagram showing the relation between average Md values andvariations of the Ac₁ point of the alloy wherein 1 mol. % of any one ofalloying elements is added to iron.

FIG. 4 is a diagram showing the relation between average Md value andδ-ferrite phase volume,

FIG. 5 is a diagram showing the relation between average Md value andaverage Bo value (hereinafter designated as "Average Md--Average Bodiagram"), wherein the process of development of 9-12% chromium boilersteels is shown,

FIG. 6 is a diagram showing the relation between average Md value andaverage Bo value specific to the heat resistant steels according to thisinvention,

FIG. 7 is a diagram showing the relation between allowable stress andaverage Bo value for the 9-12% chromium boiler steels,

FIG. 8 is the Average Md--Average Bo diagram, wherein the process ofdevelopment of 9-12% chromium turbine steels is shown,

FIG. 9 is a diagram showing results of Varestraint test for B-seriesspecimens of the Example.

BEST MODE FOR EXECUTING THE INVENTION

The most significant feature of the method of this invention is to firstcalculate "alloying parameters" for each alloying element in bodycentered cubic (hereinafter designated as "bcc") crystal structure ofiron base alloys using DV-Xα cluster method which is one of themolecular orbital calculating methods, and then clarify the action andfunction of each said alloying element in terms of the alloyingparameters, and finally select types of alloying elements and theircontents both of which are capable of giving desired properties to thealloys.

By using the above-mentioned alloying parameters, phase stability andhigh temperature creep properties of the ferritic heat resistant steelcan be estimated. That is to say, theoretical estimation of the ferriticheat resistant steel can be made, which leads to further developing ofnew heat resistant steels.

The above-mentioned heat resistant steels (3) to (5) having the novelchemical compositions are the steels designed according to the method ofthis invention.

Now, the fundamental theory of the method of this invention will bedescribed in detail.

I! Induction of Alloying Parameter by Molecular Orbital Method

FIG. 3 shows a cluster model used for a calculation of the electronicstructure of a bcc iron alloy. In this model, a center positionedalloying element M is surrounded by 14 iron atoms in the first and thesecond nearest neighbor positions. Inter-atomic distance in the clusteris determined on the basis of the lattice constant of pure iron, i.e.,0.2866 nm, and an electronic structure of the alloy in the case ofreplacing the center positioned iron atom with any alloying element M iscalculated by the DV-Xα cluster method (Discrete-Variation-Xα clustermethod, the details of which are described in "¹ The Fundamentals toQuantum Material Chemistry", published by Kyoritsu Shuppan K.K. . . .Reference 4, and Japanese Patent Publication No.5-40806) which is one ofthe molecular orbital calculating methods.

Values of two types of alloying parameters for several alloying elementsobtained by the calculation are shown in Table 3. One of those alloyingparameters is Bond Order (abbreviated as "Bo") which represents thedegree of overlapping of electron clouds caused between iron atoms andthe M atom. The greater is the Bo value, the stronger is theinter-atomic bond. The other alloying parameter is d-orbit energy level(abbreviated as "Md") of alloying element M, which is correlative withthe electronegativity and the atomic radius of the alloying element.Although the unit of Md is electron volt (eV), description of this unitis hereinafter omitted for simplification.

Md values for non-transition metal elements, i.e., carbon, nitrogen andsilicon, as shown in Table 3, were determined on the basis of phasediagrams and experimental data. Since these elements do not haved-electrons, they are handled in the above-mentioned manner to discusson the same basis as the transition elements.

Average content is determined for each alloying element, as shown in thefollowing formulae and average Bo and Md values are calculated on thebasis of each said average content of the element.

    Average Bo value=ΣXi(Bo)i                            1

    Average Md value=ΣXi(Md)i                            2

wherein Xi is molar fraction of an element "i", (Bo)i is Bo value of theelement "i" and (Md)i is Md value of the element "i". In reality Bo andMd values cited in Table 3 are used in place of those average values.Both Bo and Md values not cited in Table 3 are regarded as zero.

II! Estimation of Feature of Element and Selection of Alloying Elementson the Basis of the Alloying Parameter

Alloying parameters of elements (M) are arranged and illustrated on theAverage Bo--Average Md diagram in FIG. 2, wherein average Bo and averageMd of every "Fe-1 mol % M alloys" are marked with symbol . It will beapparent from the diagram that the positions of symbol  are greatlychanged by the types of alloying elements. Every alloying element, whosesymbol  is located in the upper-right zone of symbol ∘ of iron, is aferrite former except manganese. Manganese and other alloying elementswhich are located in the lower-left zone in FIG. 2 are austeniteformers.

It is preferable that the alloying elements of the ferritic heatresistant steel have a higher Bo value and a lower Md value. The high Boelements strengthen the alloy by increasing the inter-atomic bond. Md isconnected with phase stability of the alloy as hereinafter described. Ifthe average Md value of the alloy is increased, the secondary phase (δphase, etc.) is unfavorably precipitated in the matrix (refer to "Ironand Steel" vol. 78, (1992), p.1337 . . . Reference 5). In view of highaveraged-Bo value and low averaged-Md value, chromium is an optimumalloying element which well satisfies those conditions as illustrated inFIG. 2. Chromium exhibits the highest inclination of "alloying vector,"i.e., the ratio of "average Bo/average MD". The ratio with respect toeach element decreases in the order of Mo, W, Re, V, Nb, Ta, Zr, Hf andTi.

On the other hand, austenite forming elements except manganese exhibit anegative "average Bo/average Md" ratio, which decreases in the order ofCo, Ni and Cu. As shown in Tables 1 and 2, most of the boiler steels donot contain nickel, whereas most of the turbine steels contain it as anessential element. Copper is contained in only the HCM12A steel forboilers. Cobalt is not contained in any of the turbine and boilersteels.

Rhenium, as well as cobalt, has not been used intentionally in spite ofthe fact that they seem to be effective alloying elements for ferriticheat resistant steels in view of the above-mentioned theoreticalpresumption. Ferritic heat resistant steels according to this inventioncontain cobalt, or cobalt and rhenium as essential components asdescribed hereinafter.

Ferritic heat resistant steels are usually tempered to obtain a singlephase structure of tempered martensite. In order to increase creeprupture strength at an elevated temperature for long periods of time, atempering treatment should be carried out at a temperature as high aspossible. For this purpose, the Ac₁ transformation point which is theupper limit of the tempering temperature must be elevated. The Ac₁transformation point is given by the following empirical formula:

    Ac.sub.1 point (° C.)=760.1-23.6Mn-58.6Ni-8.7Co-6.0Cu+4.2Cr+25.7Mo+10.3W+84V3

wherein each element represents content (mass %) thereof.

FIG. 3 shows a relationship between the average Md and changes of theAc₁ point (Δ Ac₁), when bcc iron is added with 1 mol. % of alloyingelements. As mentioned above, elements having a low average Md andserving to elevate the Ac₁ point are most suitable for the alloyingelement of the heat resistant steel. In this respect, FIG. 3 teachesthat vanadium having a comparatively great "Δ Ac₁ /average Md" ratio isan effective element. On the contrary, chromium scarcely contributes toelevate Δ Ac₁. In comparison with nickel and cobalt, the latter does notlower so distinctively the Ac₁ point. In this connection, cobalt isconsidered to be more suitable than nickel as an alloying element.

Since manganese lowers the Ac₁ point and does not have so great a Bovalue, the manganese content is preferably low. As copper lowers the Ac₁point of a steel to a similar degree as cobalt, addition of copper to asteel is actually tried for example in the HCM12A steel as listed inTable 1.

III! Evaluation of Phase Stability of Ferritic Heat Resistant Steels

In order to improve creep properties and toughness of the ferritic heatresistant steels, formation of δ-ferrite must be suppressed. Accordingto the method of this invention, formation of the δ-ferrite can bepredicted with fair accuracy.

FIG. 4 illustrates a correlation of amounts of residual ferrite inseveral steel specimens containing different levels of nickel andnormalized at 1050° C. with a parameter of average Md value. Theδ-ferrite phase begins to form at the average Md value slightlyexceeding 0.852 and increases in proportion to the increasing average Mdvalue. The average Md value tends to become slightly higher above theδ-ferrite forming boundary due to the addition of nickel, which is oneof the austenite stabilizing elements, to the steel.

An amount of the δ-ferrite phase can be predicted from a composition ofa steel, and whereby formation of the δ-ferrite can be suppressed. Thus,the prediction of the δ-ferrite amount on the basis of the average Mdvalue is very useful to design novel ferritic heat resistant steels.Additionally, formation of Laves phase (Fe₂ W, Fe₂ Mo, etc.) can also bepredicted, if nickel, which promotes the formation of the Laves phase,is not contained in the steel.

VI ! Evaluation of Conventional Ferritic Heat Resistant Steels

(i) Boiler Materials

Average Bo and average Md values are calculated from compositions of9-12% chromium boiler steels listed in Table 1, and plotted on theAverage Bo--Average Md diagram in FIG. 5.

The average Bo value of 2·1/4Cr-1% Mo steel (JIS STBA24), which is oftencompared with 9-12% chromium boiler steels, is 1.7568 and the average Mdvalue is 0.8310. These values are quite small as compared with that ofmaterials listed in FIG. 5, and accordingly cannot be illustratedtherein by the same scale.

As described in the above-mentioned reference 1, 9% Cr steel wasdeveloped in the order of T9→T91→NF616. T91 (modified 9Cr-1Mo) is asteel which was developed by adding optimum amounts of vanadium andniobium, which are carbide or carbo-nitride forming elements, to T9(9Cr-1Mo). NF616 is a steel which was developed by decreasing the amountof molybdenum and adding tungsten in place of molybdenum, which exhibitsthe highest creep rupture strength at present among other 9% Cr steelshitherto produced.

Development of 9% Cr steel will be understood in view of increase ofboth Bo and Md values as shown by arrow marks on the Average Bo--AverageMd diagram in FIG. 8. The average Md value of NF616 is 0.8519, whichcorresponds to the average Md value at a boundary of δ-ferrite phaseformation in the case that nickel is not contained. Thus, NF616 is saidto be an alloy which is strengthened by adding thereto certain alloyingelements in as high as possible amounts as not to cause δ-ferrite phaseformation. It is considered that steel superior to NF616 will not beattainable in the series of steels which do not contain any austenitestabilizing elements, such as nickel and cobalt.

12% Cr steel was developed in the order of HT9→HCM12→HCM12A. HCM12A is asteel which was developed by decreasing the amount of carbon in HT9 andadding thereto tungsten and niobium. Amounts of molybdenum and tungstenin HCM12A are controlled so that the molybdenum equivalent Mo+(1/2)W !may descend below 1.5%. As mentioned above, formation of the δ-ferritephase is suppressed by adding 1% copper to the steel.

Development of 12% Cr steels have followed a zigzag line as illustratedon the Average Bo--Average Md diagram in FIG. 5. The average Md value ofHCM12A is 0.8536, which approximately corresponds to the average Mdvalue at a boundary of δ-ferrite phase formation, but is somewhat higherthan the boundary. Since HCM12A contains 1% copper which is an austeniteformer like nickel and cobalt, the boundary average Md value is slightlyelevated. The average Md value of the steel containing 1% copper isconsidered to be 0.853 to 0.854. HCM12A is therefore said to be a steelwhich aims at a critical composition as not to cause δ-ferrite phaseformation. When subjecting the steel to a heat treatment slightlydifferent from the standard, formation of the δ-ferrite phase will beduly expected.

More than 30 vol. % of δ-ferrite is formed in HCM12 steel, since it hassuch a high average Md value as 0.8606 and does not contain anyaustenite forming elements. As far as TB12 steel is concerned, theδ-ferrite phase would be formed therein in view of its high average Mdvalue (0.8594). It is well known that the δ-ferrite phase is similarlyformed in EM12, Tempaloy F-9, HCM9M and the like 9% Cr steels havinghigh average Md values.

It will be summarized that NF616, HCM12A and the similar recentlydeveloped materials exhibit a structure of single phase martensitewithout δ-ferrite and have a great bond order value. B1-B5 steels markedby □ symbol in FIG. 5 are exemplified ferritic heat resistant steels ofthis invention mentioned later (the heat resistant steels of theabove-mentioned (3)), and the average Md values and average Bo values ofthese steels are in a area surrounded by a parallelogram.

FIG. 6 is an enlarged view of the parallelogram area in FIG. 5, whereinsegment AB is expressed as Average Bo=2.7907× (Average Md)-0.5727,segment DC is expresses as Average Bo=2.7907× (Average Md)-0.5908 andcoordinates of points A, B, C and D are expressed as follows:

point A . . . average Md value=0.8563, average Bo value=1.817

point B . . . average Md value=0.8520, average Bo value=1.805

point C . . . average Md value=0.8585, average Bo value=1.805

point D . . . average Md value=0.8628, average Bo value=1.817

FIG. 7 shows a relationship between allowable stress at 600° C.(ordinate) and average Bo value (abscissa), wherein the δ-ferrite phaseis formed in alloys marked by □ symbol and not in alloys marked by symbol. Allowable stress of alloys in which the δ-ferrite phase is notformed is known to linearly increase along a straight line in proportionto the average Bo value. On the other hand, allowable stress of alloysin which δ-ferrite is formed is generally low and lies in a zone belowsaid line. Although the δ-ferrite phase in a steel may be effective toincrease its weldability, formation of the δ-ferrite phase should besuppressed in the case that the allowable stress is desired to increase.

(ii) Turbine Materials

ii-1 Rotor Materials

Development of 9-12% chromium turbine steels (refer to Table 2) is alsodescribed in Reference 1. The rotor materials have been developed in theorder of "H46 for small sized article" →GE→TMK1→TMK2. GE for large sizearticles was developed from H46 by modifying it in respect of loweringniobium content below 0.1% and chromium content below 10% in order toinhibit a formation of abnormal segregation (segregation of δ-ferritephase, MnS and coarse NbC) in a large scale ingot upon solidification.TMK1 was developed from GE by lowering its carbon content and increasingits molybdenum content. TMK2 was further developed from TMK1 by loweringits molybdenum content and increasing its tungsten content in order toincrease its creep rupture strength.

Development of 12% chromium steel is illustrated on the AverageBo--Average Md diagram in FIG. 8. The locations of the exemplifiedsteels (T1-T5) of this invention are shown by □ symbols in FIG. 8, andthe average Md values and the average Bo values of the ferritic heatresistant steels of this invention (heat resistant steels of theabove-mentioned (3)) are in a zone surrounded by the parallelogram.

H46 was changed into GE by greatly lowering the average Md value as wellas the average Bo value. It can be understood that the segregation hasbeen avoided thoroughly in the production of large scale rotors.However, the development of the rotor materials in the order ofGE→TMK1→TMK2 is based on increase of both the average Md value and theaverage Bo value. This is similar to the change of the boiler materialsin the order of T9→T91→NF616. It could be said that the average Md valueof each of the rotor materials, GE, TMK1 and TMK2, eventually came nearto that of H46, as a result of aiming at improvements of the properties.

Thus, TMK1 and TMK2 were developed, each having the average Bo valuehigher than that of H46. The average Bo value and average Md value ofTMK2 were 1.8048 and 0.8520, respectively, and these values have turnedout to be very near the average Bo value of 1.8026 and the average Mdvalue of 0.8519 of NF616, respectively. That is to say, the average Bovalues of both boiler and turbine materials are brought together inalmost the same zone, as well as the average Md values of bothmaterials. Since TMK1 and TMK2 contain 0.5-0.6% nickel, the average Mdvalues on the δ-ferrite forming boundary is about 0.855 (refer to FIG.4).

An alloy developed for producing turbine rotor members, which will beexposed to attack of water vapor at a super high temperature such as593° C., is now subjected to a demonstration test for a super hightemperature steam turbine, held at Wakamatsu Power Plant, and the creeprupture strength of the alloy test specimen kept at 593° C. for 100,000hours may be 12.4 kgf/mm² (122 MPa), which is near that of TMK1.Actually, the location of the average Bo value--average Md value of thisalloy (designated as "Wakamatsu Rotor") on the Average Bo--Average Mdvalue diagram (FIG. 8) is very near to that of TMK1. The alloy(Wakamatsu Rotor) was developed from TAF by selecting optimum amounts ofcarbon and nitrogen. Another 12% Cr series heat resistant steel durablefor a super-high temperature of 593° C. was recently developed from GE.The creep rupture strength of the alloy specimen kept at 593° C. for100,000 hours is 15.3 kgf/mm² (150 MPa) which is slightly higher thanthat of "Wakamatsu Rotor". However, location (shown by "A") of this heatresistant steel on the Average Bo--Average Md diagram is on the low Mdside as compared with that of TMK2.

ii-2 Cast Steels

Cast steels are suitable for producing a turbine chamber, a blade ringand similar turbine members. However, the conventional 2·1/4Cr-1Mo caststeel is poor in high temperature strength and accordingly can not beused in a steam atmosphere higher than 593° C. Table 4 showscompositions of several 9-12% Cr cast steels developed by differentsteel makers. Locations of these heat resistant steels on the AverageBo--Average Md diagram are on the low average Bo and low average Md areaas compared with the rotor materials, as apparent from FIG. 8. Thereason is that the composition of the steel is controlled in a manner toavoid segregation and formation of the δ-ferrite phase in the caststeel. Among these cast steels, TSB12Cr is very similar to MJC12 and T91cast steel and already utilized in the Kawagoe No. 1 and No. 2 plants.Although MHI12Cr was already used in the above-mentioned demonstrationtest for a super high temperature turbine, held at Wakamatsu, theaverage Md value is low and seems to be designed for avoiding thesegregation. On the other hand, HITACHI 12Cr exhibits higher average Mdand higher average Bo values than other 12Cr steels.

As particularly described above, specific properties of each alloy arefairly clarified in view of the Average Bo--Average Md diagram. It willbe understood to one skilled in the art that the development of theconventional materials can be outlined on this diagram, and besides,novel ferritic heat resistant steels provided with more excellentproperties than ever can be predicted and designed using this diagram.

V! Optimum Range on the Average Bo--Average Md Diagram

Areas surrounded by the parallelograms as shown in FIGS. 5 and 8 and theenlarged area in FIG. 6 are the optimum range for the heat resistantsteels. The segment BC shows an average Bo level of 1.805, and if theaverage Bo decreases below the segment level, the creep properties areworsened (refer to FIG. 7). The segment AD is the average Bo level of1.817, and it will be actually impossible to elevate the average Bovalue above the segment level unless the phase stability is decreased.

Point D on FIG. 6 is the point at which the average Md value is 0.8628,which is the safe upper limit not to form δ-ferrite in the actualproduction of the material. It is not preferable to lower the Bo and Mdvalues below the point B (average Bo value:1.805, average Md value:0.8520) in order to maintain the high temperature properties of thealloy.

It is therefore recommendable to design a composition of a ferritic heatresistant steel so that the average Bo value is in the range of 1.805 to1.817 and the average Md value is in the range of 0.8520 to 0.8628, inthe production of steel which is excellent in high temperature creepproperties.

The direction of the segment AD in FIG. 6 and that of the segment CD aresimilar to the direction of the alloying vector of chromium, vanadium,tungsten, niobium, tantalum, rhenium, manganese and cobalt, as shown inFIG. 2, and it will be seen that if the average Bo value is elevated,the average Md value is also elevated along the direction of thealloying vector. This means that the heat resistant steel (steels ofthis invention mentioned above in item (3)) surrounded by segments AB,BC, CD and DA may be the most desirable ferritic heat resistant steels.The range of chromium content and that of carbon content of this steelare able to ensure and keep the essential physical and chemicalproperties of the steel. 0.5% of cobalt is a minimum amount to avoidformation of the δ-ferrite phase. On the other hand, if the cobaltcontent exceeds 4.3%, no further distinctive improvement of the creepproperties is expected.

Cobalt contents should be in the range of 0.5 to 4.3%, since cobaltlowers the Ac₁ transformation point. Tungsten, exhibiting the high Bovalue, is an essential element for improving high temperature creepproperties, and at least 0.5% tungsten is necessary for this purpose.However, addition of excess amounts of tungsten to the steel isdetrimental to the oxidation resistance and creep properties of theresultant steel due to the fact that Laves phase tends to be formed andthe steel is thereby embrittled. The upper limit of the tungsten contentis determined to be 2.6%. Alloying elements other than indispensableelements should be selected so that the steel can be in the optimum area(the area surrounded by the parallelogram) in FIG. 6. Although nickel isan incidental impurity and preferably as low as possible, contaminationof the steel with nickel cannot be avoided since nickel bearing scrapsare used in the production of the steel. Contents of up to 0.40% nickelis allowable.

VI! Guideline for Embodiment of this Invention

The chemical composition of the ferritic heat resistant steel will bedesigned according to the following guidelines of this invention on thebasis of the theory and empirical rules hereinbefore described.

1) Suppress formation of δ-ferrite which is detrimental to hightemperature creep properties, the δ-ferrite being suppressed to improvethe toughness and creep properties.

2) The Ac₁ transformation point shall be elevated as high as possible toimprove the creep properties.

3) A proper range of average Md values shall be selected in view of theabove-mentioned items 1) and 2). As shown in FIG. 4, the average Mdvalue is required not to exceed 0.8540 when the nickel content is notmore than 0.40%. However, the average Md value can be increased up to0.8628 by increasing the cobalt content as high as around 4%.

4) There is a relationship between the creep properties and the Bondorder (average Bo) as shown in FIG. 7. The higher is the Bo value, thehigher is the melting point of the material, resulting in an improvementof the creep properties. Therefore, the chemical composition of thesteel shall be selected in such a range that the δ-ferrite phase is notformed, i.e., the average Md value does not exceed 0.8628, and the Bovalue becomes the highest possible value.

5) In view of preceding items 1) to 4), the essential guideline is toselect such a chemical composition of the alloy that the average Bovalue is restricted in a range of 1.805 to 1.817 and the average Mdvalue is restricted in a range of 0.8520 to 0.8628.

In addition to that, guidelines for designing compositions of heatresistant steels for boiler and turbine are as follows.

6) Cobalt, one of the austenite stabilizing elements, is indispensablyadded to the steel, and, if more improvement of high temperaturestrength and phase stability is required, rhenium could be furtheradded.

7) Contents of tungsten, molybdenum, vanadium, niobium, rhenium andcobalt shall be optimized on the basis of the average Bo value andaverage Md value.

Steels manufactured according to those guidelines are the heat resistantsteels No.1 and No.2, respectively, in Table 5. The No.1 steel exhibitsfar more excellent high temperature strength than the conventionalmaterials, and is suitable for use in turbine members. This type ofsteel is hereinafter designated as T-series steel. On the other hand,the No. 2 steel exhibits high temperature creep strength and excellentweldability, and is suitable for use in boiler members. The latter typeof steel is hereinafter designated as B-series steel.

VII ! High Strength Ferritic Heat Resistant Steels of this Invention

Table 5 shows compositions of ferritic heat resistant steels(above-mentioned No.1 and No.2 steels) of this invention. These steelsare designed to have a novel composition and more excellent chemical andphysical properties than that of the above-mentioned TMK2 and NF616which have the highest quality and performance for use in turbine andboiler members, respectively, at present.

While the TMK2 turbine steel contains low amounts of nickel, the steelof this invention contains cobalt instead of nickel. If the cobaltcontent is undesirably low, the δ-ferrite phase tends to be formed inthe steel. The cobalt content is therefore restricted in a range of 0.5to 4.3%, as mentioned above.

Rhenium is an element which has a great "average Bo/average Md" ratio asshown in FIG. 2 and improves the strength of the steel withoutdiminishing the phase stability. Although only 0.01% rhenium content iseffective to strengthen the steel, more than 0.1% rhenium content ispreferable to ensure that effect. However, more than 3% rhenium contentis detrimental to the phase stability of the steel, and besides it isnot economical to make the steel because rhenium is an expensiveelement.

The chromium content is adjusted so as to increase both the average Mdand the average Bo values of the steel as high as possible, to an extentnot to form the δ-ferrite phase.

Now, a composition of the No.1 steel (mainly used in turbine members)and that of the No.2 steel (mainly used in boiler members) will bedescribed in more detail.

(i) No.1 Steel (T-series Steel)

This steel is typically used in manufacturing turbine members (rotors,blades and some other cast parts. The composition of the steel ispreferably adjusted to exhibit both low average Bo and Md values whenthe steel is cast) and also in automotive and aeroplane engine parts.

1) This steel is designed to contain therein 0.5˜4.3% cobalt. Theability of cobalt to stabilize the austenite phase is about half that ofnickel. The average Md value at the δ-ferrite phase appearing boundaryis therefore anticipated as 0.860 when the cobalt content is 3.0%. Theseaverage Md values correspond to the value at the δ-ferrite phaseappearing boundary when the nickel content is 1.5% as shown in FIG. 4.

The ability of cobalt to lower the Ac₁ point is far less than that ofnickel, as apparent from the foregoing formula 3. If cobalt is added tothe steel instead of nickel, the Ac₁ point can be kept at a higher levelwhich brings about such an advantage that the steel can be tempered at ahigh temperature.

Thus, nickel which tends to reduce creep properties of a steel is, inprinciple, replaced with cobalt in the steels of this invention. Sincesuch steels are produced using partly nickel bearing steel scraps foreconomical reasons, some contamination of the steels cannot be avoidedin spite of the fact that the lowest nickel content is preferable. Theallowable upper limit of the nickel content of the steels of thisinvention is therefore restricted to 0.40%, in view of both practicalnecessities and conditions for δ-ferrite phase formation. The upperlimit of the nickel content is preferably 0.25%.

2) In order to adjust the average Md value, the content of nitrogen,which has a negative Md value, is restricted in a range of 0.01 to0.10%.

3) The allowable upper limit of the manganese content is restricted to0.45%. A low manganese content together with a low silicon content hasan effect of suppressing embrittlement of the steel derived fromsegregation of impurity elements at grain boundaries and embrittlementderived from precipitation of carbides, resulting in a quite lowembrittlement sensitivity. The lower limit of the manganese content istherefore substantially zero.

4) Rhenium is a preferable alloying element for the ferritic heatresistant steel, as shown in FIG. 2. However, since rhenium is a veryexpensive element, it can be used when its addition is absolutelynecessary. In order to ensure the function of rhenium for improving thetoughness of the steel against fracture, at least 0.01%, preferably atleast 0.1% rhenium should be added thereto. The upper limit of theamount of rhenium is determined to be 3.0% for the above-mentionedeconomical reasons.

Suitable molybdenum and tungsten contents in the steel are influenced bythe rhenium content for technical reasons hereinafter described. Thelower limit of the molybdenum content is determined to be 0.02%. Thetungsten content preferably ranges from 1.0 to 2.0%. As alreadydescribed in item V!, excess amounts of tungsten may be detrimental tovarious properties of the steel. Accordingly, a part of the tungsten ispreferably replaced with rhenium which is innocuous to the steel.

5) Boron is often added to ferritic heat resistant steels in order toimprove the hardenability and refine the steel structure as describedhereinbefore. Boron could be added to the steel of this invention whenfurther increase in high temperature strength and toughness is required.In order to increase the high temperature creep strength, addition ofmore than 0.001% boron is preferable. However, since more than 0.02%boron is injurious to the workability, the upper limit of boron contentshould be 0.02%.

6) The chromium content is so determined that the average Bo value andaverage Md value of the steel are increased to the highest possiblelevel.

7) Silicon is used as a deoxidizer for the steel. Since silicon reducesthe toughness of the steel, the residual silicon amount in the steel ispreferably as low as possible, and may be substantially zero. The upperlimit of the silicon content is determined to be 0.10%. Althoughaluminum can also be used as a deoxidizer for the steel, it forms A1Nand reduces the function of nitrogen. The content of aluminum in theform of acid soluble aluminum may preferably be less than 0.02%. Bothphosphorus and sulfur, being incidental impurities, are restricted below0.01%, respectively, and should be as low as possible to keep clean thesteel structure.

(ii) No.2 Steel (B-series Steel)

This steel is principally used in boiler members exposed to anenvironment of high temperature and high pressure water vapor and alsoin heat exchanger tube members in chemical or other industries. Theguidelines for designing these steel compositions will be specifiedbelow.

1) In order to stabilize the austenite phase, 0.5-4.3% cobalt iscontained in the steel. The average Md value at the δ-ferrite phaseforming boundary is predicted to be 0.856 at 1.5% cobalt content, 0.858at 2.5% cobalt content and 0.860 at 3.0% cobalt content (the same asthat in the No.1 steel). These average Md values correspond to theaverage Md values at the δ-ferrite phase forming boundary at 0.75%nickel, 1.25% nickel and 1.5% nickel, respectively, as in FIG. 4. Nickelis not positively added to the B-series steel. The upper limit of thenickel content which is allowable to the steel is 0.40%, and preferably0.25%, the same as in the T-series steel.

2) Rhenium is added to the B series steel if it is necessary, the sameas in the No.1 steel. If rhenium needs to be added to the steel, itscontent should be more than 0.01%, preferably more than 0.1%. The upperlimit of the rhenium content is 3.0%. Suitable molybdenum and tungstencontents are influenced by the rhenium content. That is to say, thecomposition of the No.2 steel, when including rhenium, is adjusted bycontrolling the molybdenum and tungsten contents, the same as in theNo.1 steel. Alloying vectors of rhenium, molybdenum and tungsten havesubstantially the same direction on the Average Bo--Average Md diagramin FIG. 2, and the influence caused by addition of rhenium can bereduced by lowering the molybdenum and/or tungsten contents. Themagnitude of the alloying vector of rhenium is smaller than that ofmolybdenum and tungsten. The average Bo value and average Md value cantherefore be maintained at their original values by slightly reducingthe amounts of molybdenum and/or tungsten and substantially increasingthe amount of rhenium instead. The favorable tungsten content is thesame as that in the steel No.1.

3) The chromium content is determined to be such values that the averageBo value and the average Md value may be as high as possible. As thechromium content increases, Ac₁ point of the steel is elevated,resulting in improvement on creep properties.

4) Silicon is used as a deoxidizer also for the B-series heat resistantsteel. Oxidation of boiler steel by an attack of high temperature watervapor is a serious problem to be solved. Silicon in the steel iseffective to suppress the oxidation of the steel. In view of thisoxidation suppressing effect, as well as an effect of decreasingtoughness and high temperature creep strength, the maximum siliconcontent in the steel No.2 is restricted to 0.50%.

5) Handling of manganese, aluminum, nitrogen and boron and otherincidental impurities is similar to that in the steel No. 1. In order toimprove weldability of the steel No.2, the carbon content is restrictedto a level lower than that of the steel No.1.

EXAMPLE

1. Preparation of Test Specimens

(1) T-series Steel Specimens

Six steels having different compositions as shown in FIG. 14 were meltedin a high frequency vacuum induction furnace and cast into six ingotseach having a weight of 50 kg. Each ingot was heated to a temperature of1170° C., hot forged into a billet having a 130 mm thickness and a 35 mmwidth. The obtained billet was normalized by keeping it at 1100° C. for5 hours and then air cooled, followed by an annealing treatment whereinthe billet was kept at 720° C. for 20 hours and then air cooled.

After that, the following heat treatment steps simulate the heat cyclesuffered by the center zone of an actual turbine rotor.

1 keeping at 1070° C. for 5 hours and oil quenching (hardening)

2 keeping at 570° C. for 20 hours and air cooling (first tempering)

3 keeping at T °C. for 20 hours and air cooling (secondary tempering)

Specimen "T0" is the aforesaid conventional heat resistant turbine rotorsteel TMK2 which is used as a reference specimen for the variousfollowing tests. These steels are principally used in turbine membersand referred to as T-series steels.

As shown in FIG. 14, the T-series steels of this invention contain 3 acobalt. Among them, T1 and T2 steels contain about 0.9% rhenium, and T5steel contains about 1.7% rhenium. The average Md value and average Bovalue of the steels are shown in Table 7. The locations of these steelson the Average Bo-Average Md diagram are shown in FIG. 8 by □ symbol.All these specimens T1-T5 are in a higher average Bo and Md zone incomparison with the TMK2 specimen.

The Ac₁ points and AC₃ points of TMK2 and T1-T5 specimens are listed inTable 7 as well as the average Md and Bo values. Since the Ac₁ points ofT1-T5 steels of this invention are higher than that of TMK2 steel by 14°to 32° C., it can be predicted that these steels have excellent hightemperature properties.

(2) B-series Steel Specimens

Six steels having different compositions as shown in Table 6 were meltedin a high frequency vacuum induction furnace and cast into six ingotseach having a weight of 50 kg. Each ingot was heated to a temperature of1150° C., hot forged into a heavy plate having a 50 mm thickness and a110 mm width. The obtained plate was cut into about 300 mm length pieceswhich were then heated at 1150° C., and hot rolled to prepare a sheethaving 15 mm thickness and 120 mm width. The sheet was further kept at1050° C. for 1 hour and then air cooled to obtain a test specimen havinga normalized structure.

Specimen "B0" in Table 6 is the above-mentioned conventional boilersteel NF616 which is utilized as a reference specimen for the followingtests. Steels of B1-B5 are No.2 heat resistant steels designed accordingto this invention. These steels are principally used in boiler membersand referred to as B-series steels.

The B-series steels take three levels of cobalt contents, i.e., about1.5% (B1 and B2 steels), about 2.5% (B3 and B4 steels) and about 3% (B5steel). The B2, B4 and B5 steels contain rhenium. The average Md and Bovalues of these steels are shown in Table 5, as well as the Ac₁ pointand AC₃ point. The locations of these steels of this invention on theAverage Bo--Average Md diagram are shown in FIG. 8 by □ symbol. As isshown in FIG. 5, since all these specimens B1 to B5 are in a higheraverage Bo and Md zone as compared with the NF616 specimen, it can bepredicted that these steels have more excellent high temperatureproperties.

Locations of the average Bo value of the No.2 steels of this inventionare shown by an arrow mark in "allowable stress--average Bo value"diagram of FIG. 7. In view of the above-mentioned composition designingguidelines, it appears that the δ-ferrite phase is not formed in theB1-B5 specimens. The allowable stress value of the steel can thereforebe predicted by a straight line in the FIG. 7. B3, B4 and B5 steelspecimens are presumed to have about 98 MPa (10 kgf/mm²) allowablestress at 600° C.

2. Testing Procedure

Various tests were carried out using the above-mentioned specimens inaccordance with the following procedure.

(1) Tensile test at room temperature (common to T-series steels andB-series steels):

The tensile tests were carried out using JIS No.4 test specimens forT-series steels and using JIS No.14 test specimens for B-series steels.

(2) Visual inspection of microstructure (common to T-series steels andB-series steels):

Each specimen was etched by Vilella solution (chloric acid--picricacid--alcohol) and inspected with a microscope under 100 and 500magnification.

(3) Tensile test at an elevated temperature (common to T-series steelsand B-series steels):

High temperature tensile tests were carried out in accordance withdirections of JIS G 0567 using "I" shaped test specimens.

(4) Charpy impact test (common to T-series steels and B-series steels):

Charpy impact tests were carried out using JIS No.4 impact testspecimens.

(5) Creep rupture test (common to T-series steels and B-series steels):

Creep rupture tests were carried out in accordance with directions ofJIS Z 2272 using a round bar test specimen having 6 mm diameter and 30mm gauge length.

(6) Measuring maximum hardness of HAZ (only for B-series steels):

The maximum hardness of HAZ was measured in accordance with a directionof JIS Z 3101 using No.2 test specimens wherein a welding bead wasformed on the center zone of the test specimen. The welding conditionsfor forming the bead were as follows.

    ______________________________________    Welding rods     NF 616 rod having 4.0 mm                     diameter (prepared by Nittetsu                     Yosetsu K.K.)    Preheating temperature                     150° C.    Welding current  170 A    Welding voltage  25 V    Welding speed    15 cm/min.    Heat input       17 KJ/cm    ______________________________________

(7) Varestraint test (only for B-series steels)

Longi-Varestraint tests were carried out, wherein a welding bead wasformed on the test specimen by a TIG welding process and a shock ofbending load was applied on a point in the bead length to cause a hightemperature crack therein.

The conditions for the tests were as follows.

    ______________________________________    Electrodes used Th-W electrodes for TIG                    welding process having 3.2 mm                    diameter    Welding voltage 18-19 V    Welding current 300 A    Welding speeds  100 mm/min.    Argon gas flow rate                    15Λ /min.    Surface strain  ε = 4%    ______________________________________

(1) Tempering Test and Determination of Standard Tempering Conditions

(i) T-series Steels

The T series steels were subjected to a tensile test at room temperatureafter heat treating them at the secondary tempering temperature (T) of630° C., 660° C., 690° C. or 720° C. as hereinbefore described in 1 (1)3.

Test results are shown in Table 8. In the case that the temperingtemperature is as low as 630°-660° C., 0.2% proof stress of T3, T4 andT5 specimens and tensile strength of T4 specimens are almost equal tothat of T0, whereas in the case of high tempering temperature exceeding690° C., tensile strength and 0.2% proof stress of T3, T4 and T5specimens are much higher than that of T0 (TMK2). Tensile strength and0.2% proof stress of T1 and T2 specimens are higher than that of T0(TMK2) at any tempering temperature. T1 specimen exhibits the maximum0.2% proof stress. It is apparent from FIG. 8 that T1-T4 specimens ofthis invention exhibit excellent resistance to temper softening higherthan that of the reference specimen T0due to the action of chromium andcobalt.

(ii) B-series Steel

The above-mentioned normalized specimens according to 1 (2) were heatedat 670° C., 700° C., 730° C., 780° C. or 800° C. for 3 hours, and thentempered by air cooling treatment thereby preparing specimens for a roomtemperature tensile test. The test results are shown in Table 9.

Tensile strength and 0.2% proof stress of the reference specimen B0(NF616) are the lowest among B-series steel specimens at any temperingtemperature and the values of the B-series specimens increase in theorder of "B1 and B2", "B5" and "B3 and B4". The B1-B4 specimens exhibitexcellent resistance to temper softening due to the action of chromiumand cobalt, as compared with that of the reference specimen B0. Table 9shows the action of rhenium as well.

In view of the test results in Table 8 and 9, a standard temperingtreatment for the various test specimens was determined as follows.

Standard tempering treatment for T-series steels:

keeping at 680° C. for 20 hours and air cooling

Standard tempering treatment for B-series steels:

keeping at 770° C. for 1 hour and air cooling

(2) Evaluation of the Standard Tempered Specimen

The standard tempered specimens of T-series and B-series steels weresubjected to following various tests.

(i) Tensile Test at Room Temperature:

The test results of room temperature tensile tests are shown in Table10. The T-series steels of this invention exhibited tensile strengthhigher than that of the reference specimen T0, and likewise the B-seriessteels of this invention exhibited tensile strength higher than that ofthe reference specimen B0. Elongation to rupture of the T-series andB-series steels were about 20%, and they are strong enough.

(ii) Tensile Test at Elevated Temperature:

The test results of high temperature tensile tests are shown in Table11. The tensile strength and 0.2% proof stress of each specimen at 600°C. have a similar tendency to that at room temperature. Both T-seriessteels and B-series steels exhibited higher tensile strength than thatof the reference test specimens T0 and B0, respectively, as well aselongation to rupture and reduction of area to rupture.

By adding cobalt to the steel, the amount of chromium, which iseffective to improve corrosion resistance, can be increased, and furtherimprovement of the tensile strength of the steel can be obtained.Rhenium has a complementary effect on the action of molybdenum andtungsten, and seems to increase toughness of the resultant steel ashereinafter described. By addition of both cobalt and rhenium, theresultant steel can be excellent in corrosion resistance, as well astensile strength and toughness, as compared with the reference specimen.

(iii) Charpy Impact Test:

Table 12 shows a ductile-brittle transition temperature (FATT) of theT-series steels. As described hereinafter, as the high temperature creepstrength increases, the FATT is elevated. However, the extended range ofFATT does not cause any problems in the actual use of the T-seriessteels.

Table 13 shows energy absorption of B-series steel specimens at 0° C.,all of which exceed 10 kgf·m. These values are high enough to meet therequirements of the boiler material.

(iv) Visual Inspection of Microstructure:

All test specimens of T-series and B-series steels exhibited a temperedmartensitic structure. The δ-ferrite phase was scarcely found in thespecimens.

(v) Results of Creep Rupture Test:

Results of creep rupture tests for T-series and B-series steels carriedout at 650° C. are shown in Table 14 and 15, respectively. It isapparent from the Tables that both T-series and B-series steels of thisinvention are excellent in creep rupture properties as compared with thereference specimens (T0, B0). Particularly, T-series steel of thisinvention exhibited excellent creep rupture properties among otherconventional turbine steels hitherto developed in and outside Japan.

Seven different creep rupture tests with different conditions wereapplied to each steel specimen, and, on the basis of the test results,the creep rupture strengths of the steel specimens which were kept atseveral temperature levels for 100,000 hours were obtained by aninterpolating method using the Larson-Miller parameter. The specimentest temperature levels were 580° C., 600° C., 625° C. and 650° C. forT-series steel specimens and 600° C., and 625° C. for B-series steelspecimens. The test results are shown in Table 16 and Table 17, whereinthe creep rupture strength of both the T-series and B-series steelspecimens of this invention are distinctively higher than that of thereference specimens (T01, B01).

(vi) Measuring Maximum Hardness of HAZ:

In order to investigate susceptibility to low temperature crackformation of B-series steel upon welding, the maximum hardness of HAZwas measured. The test results are shown in Table 18, wherein all thetest specimens exhibited 410-420 Hv maximum hardness, by which theB-series steel specimens are presumed to have such susceptibility to lowtemperature cracking comparable with that of the ordinary 12% Cr steel.

(vii) Results of Varestraint Test:

In order to investigate susceptibility to high temperature crackformation of the B-series steels upon welding, the above-mentionedLongi-varestraint test was executed. Total cracking lengths are shown inFIG. 9. Although the total cracking lengths of the steel specimens ofthis invention are equal to or slightly longer than that of thereference specimen (B0), they are shorter than that of T91 steel as acomparative specimen. The B series steel specimens are thereforepresumed to have such susceptibility to high temperature crackingcomparable with that of the ordinary 12% Cr steel. In view of testresults of those items (vi) and (vii), the B-series steels of thisinvention are said to be a favorable boiler material which must haveexcellent weldability.

Industrial Applicability

According to the method of this invention, a ferritic iron-base alloycan be designed on the basis of a predicting system without dependingupon a series of experimentations which require huge amounts of time,cost and labor, and in particular a ferritic heat resistant steel havingexcellent physical and chemical properties can be readily andefficiently manufactured. More particularly, the ferritic heat resistantsteel having physical and chemical properties more excellent than thatof the conventional best quality steels, as disclosed in the Examples,can be theoretically designed and actually manufactured.

The ferritic heat resistant steel of this invention also exhibits highcorrosion and oxidation resistance, in view of its chemical compositionwherein chromium is the main component. The steel of this invention istherefore widely used in heat resistant materials and corrosionresistance materials, and more particularly in members of thermal powerplant or the like energy plants which are exposed to severe water vaporattacks. Highly efficient ultra super high critical pressure powerplants have been developed in recent years for matching the globalenvironmental safeguard, and the heat resistant steel of this inventionis provided with such physical and chemical properties that it issuitable for the members of such power plants.

                                      TABLE 1    __________________________________________________________________________    Chemical Composition of 9-12% Cr Steels for Boilers (mass %, Fe:bal.)    Steels     C  Si Mn Cr Mo W V  Nb B  Others    __________________________________________________________________________    9% Cr        T9     0.12                  0.6                     0.45                        9.0                           1.0                              --                                -- -- -- --    Steels        HCM9M  0.07                  0.3                     0.45                        9.0                           2.0                              --                                -- -- -- --        Tempaloy F-9               0.06                  0.5                     0.60                        9.0                           1.0                              --                                0.25                                   0.40                                      0.005                                         --        EM12   0.10                  0.4                     0.10                        9.0                           2.0                              --                                0.30                                   0.40                                      -- --        T91    0.10                  0.4                     0.45                        9.0                           1.0                              --                                0.20                                   0.08                                      -- 0.04N        NF616  0.07                  0.06                     0.45                        9.0                           0.5                              1.8                                0.20                                   0.05                                      0.004                                         0.06N    12% Cr        HCM12  0.10                  0.3                     0.55                        12.0                           1.0                              1.0                                0.25                                   0.05                                      -- 0.03N    Steels        AMAX12Cr               0.07                  0.3                     0.60                        12.0                           1.5                              1.0                                0.20                                   0.05                                      -- --        HT9    0.20                  0.3                     0.55                        12.0                           1.0                              --                                0.25                                   -- -- --        HCM12A 0.11                  0.1                     0.60                        11.0                           0.4                              2.0                                0.20                                   0.05                                      0.003                                         0.06N, 1.0Cu        TB12   0.11                  0.6                     0.50                        12.0                           0.5                              1.8                                0.20                                   0.05                                      0.004                                         0.06N    __________________________________________________________________________

                                      TABLE 2    __________________________________________________________________________    Chemical Composition of 9-12% Cr Steels for Turbines (mass %, Fe:bal.)    Steels        C  Si  Mn Ni Cr  Mo                           W V   Nb B  N    __________________________________________________________________________    H46 0.15           0.40               0.60                  -- 12.0                         0.5                           --                             0.30                                 0.25                                    -- 0.050    GE  0.18           0.30               0.60                  0.60                     10.5                         1.0                           --                             0.20                                 0.06                                    -- 0.060    TAF 0.20           0.30               0.50                  -- 10.5                         1.5                           --                             0.20                                 0.15                                    0.03                                       0.015    TMK1        0.14           0.05               0.50                  0.60                     10.3                         1.5                           --                             0.17                                 0.06                                    -- 0.040    TMK2        0.14           0.05               0.50                  0.50                     10.5                         0.5                           1.8                             0.17                                 0.06                                    -- 0.040    MJC12        0.10           0.70               0.70                  0.50                     9.5 1.0                           --                             0.15                                 0.06                                    -- 0.040    __________________________________________________________________________

                  TABLE 3    ______________________________________    Elements              M d (eV) B o    ______________________________________    3 d       Ti          2.497    2.325              V           1.610    2.268              Cr          1.059    2.231              Mn          0.854    1.902              Fe          0.825    1.761              Co          0.755    1.668              Ni          0.661    1.551              Cu          0.637    1.361    4 d       Zr          3.074    2.511              Nb          2.335    2.523              Mo          1.663    2.451    5 d       Hf          3.159    2.577              Ta          2.486    2.570              W           1.836    2.512              Re          1.294    2.094    Others    C           -0.230   0              N           -0.400   0              Si          1.034    0    ______________________________________

                                      TABLE 4    __________________________________________________________________________    Chemical Composition of 9-12% Cr Cast Steels for Turbines (mass %,    Fe:bal.)    Steels     C  Si Mn Ni Cr Ma                                V  W  Nb N    __________________________________________________________________________    T91 MAN/GF 0.11                  0.4                     0.4                        0.2                           9.0                              0.9                                0.21                                   -- 0.08                                         0.05    T91 IHI/Okano               0.12                  0.36                     0.51                        0.07                           9.0                              0.9                                0.22                                   -- 0.10                                         0.03    TSB 12Cr (Kawagoe)               0.12                  0.50                     0.48                        0.66                           10.0                              0.8                                0.27                                   -- 0.06                                         0.05    MHI 12Cr (Wakamatsu)               0.12                  -- -- 0.5                           10.0                              0.8                                0.25                                   -- 0.06                                         0.05    Hitachi 12Cr               0.13                  0.28                     0.58                        0.58                           10.5                              1.1                                0.22                                   0.23                                      0.06                                         0.04    __________________________________________________________________________

                                      TABLE 5    __________________________________________________________________________    Chemical Composition of Ferritic Heat Resistant Steels of This Invention    (mass %, Fe:bal.)    C      N  Si  V  Cr Mn  Co Nb Mo W  B  Re    __________________________________________________________________________    No. 1        0.07-           0.01-              ≦0.10                  0.12-                     10.0-                        ≦0.45                            0.5-                               0.02-                                  0.02-                                     0.5-                                        0- 0-    T-series        0.14           0.10   0.22                     13.5   4.3                               0.10                                  0.8                                     2.6                                        0.02                                           3.0    No. 2        0.02-           0.01-              ≦0.50                  0.15-                     9.0-                        ≦0.45                            0.5-                               0.02-                                  0.02-                                     0.5-                                        0- 0-    B-series        0.12           0.10   0.25                     13.5   4.3                               0.10                                  0.8                                     2.6                                        0.02                                           3.0    __________________________________________________________________________

                                      TABLE 6    __________________________________________________________________________    Chemical Composition of Ferritic Heat Resistant Steel Specimens (weight %    Fe:bal.)    Steel No.          C  Si Mn P  S  Ni Cr Ma V  W  Nb Ca Re sol.Al                                                     B  N    __________________________________________________________________________    TMK2        T0          0.14             0.05                0.53                   0.003                      0.002                         0.54                            10.42                               0.51                                  0.18                                     1.83                                        0.06                                           --*                                              --*                                                 --**                                                     --***                                                         0.042    T   T1          0.14             0.02                0.01                   0.002                      0.003                         --*                            12.07                               0.49                                  0.17                                     1.81                                        0.06                                           3.08                                              0.92                                                 --**                                                     0.008                                                         0.042    series        T2          0.14             0.02                0.01                   0.002                      0.002                         --*                            12.58                               0.50                                  0.17                                     1.82                                        0.06                                           3.07                                              --*                                                 --**                                                     0.008                                                         0.042        T3          0.11             0.02                0.01                   0.002                      0.003                         --*                            11.05                               0.39                                  0.20                                     1.95                                        0.08                                           3.09                                              0.92                                                 --**                                                     0.008                                                         0.019        T4          0.11             0.02                0.01                   0.003                      0.002                         --*                            11.56                               0.41                                  0.20                                     1.91                                        0.08                                           3.09                                              --*                                                 --**                                                     0.008                                                         0.018        T5          0.11             0.02                0.01                   0.002                      0.002                         --*                            11.12                               0.10                                  0.20                                     1.92                                        0.08                                           3.04                                              1.69                                                 --**                                                     0.008                                                         0.020    NF616        B0          0.066             0.08                0.45                   0.002                      0.003                         --*                            9.00                               0.51                                  0.19                                     1.89                                        0.050                                           --*                                              --*                                                 0.008                                                     0.003                                                         0.049    B   B1          0.066             0.08                0.46                   0.002                      0.003                         --*                            10.04                               0.50                                  0.19                                     1.89                                        0.050                                           1.49                                              --*                                                 0.011                                                     0.003                                                         0.048    series        B2          0.065             0.08                0.47                   0.003                      0.002                         --*                            10.17                               0.53                                  0.19                                     1.60                                        0.055                                           1.54                                              0.59                                                 0.013                                                     0.003                                                         0.053        B3          0.066             0.08                0.48                   0.003                      0.002                         --*                            11.73                               0.49                                  0.19                                     1.89                                        0.050                                           2.54                                              --*                                                 0.011                                                     0.003                                                         0.053        B4          0.063             0.08                0.48                   0.004                      0.002                         --*                            11.60                               0.52                                  0.19                                     1.59                                        0.054                                           2.55                                              0.59                                                 0.013                                                     0.003                                                         0.057        B5          0.068             0.06                0.19                   0.001                      0.002                         --*                            11.65                               0.10                                  0.23                                     1.66                                        0.046                                           2.83                                              1.58                                                 0.003                                                     0.003                                                         0.046    __________________________________________________________________________     Note:     --*; less than 0.01, --**; less than 0.005, --***; less than 0.0010

                  TABLE 7    ______________________________________    Steel No.             Average Md Average Bo                                  Ac.sub.1 (°C.)                                          Ac.sub.3 (°C.)    ______________________________________    TMK2   T0    0.8519     1.804   788     886    T-series           T1    0.8555     1.812   817     910           T2    0.8554     1.813   820     890           T3    0.8560     1.811   805     863           T4    0.8558     1.812   815     882           T5    0.8559     1.811   802     877    NF616  B0    0.8526     1.803   831     955    B-series           B1    0.8542     1.806   819     947           B2    0.8544     1.807   823     940           B3    0.8574     1.814   812     940           B4    0.8572     1.813   814     937           B5    0.8575     1.814   799     917    ______________________________________

                  TABLE 8    ______________________________________    Results of Tensile Test ( T-series )          Tempering                   0.2% Proof                             Tensile                                    Rupture Reduction    Steel Temp.    Stress    Strength                                    Elongation                                            of Area    No.   (°C.)                   (kgf/mm.sup.2)                             (kgf/mm.sup.2)                                    ( % )   ( % )    ______________________________________    T0    630      84.4      98.9   19      56          660      81.1      95.2   20      56          690      76.1      89.9   21      61          720      65.4      80.4   23      64    T1    630      87.8      105.8  18      45          660      84.8      102.9  18      47          690      82.2      99.4   19      49          720      77.3      93.9   19      52    T2    630      86.7      104.5  18      45          660      84.1      101.9  18      48          690      82.0      98.9   18      48          720      78.1      94.4   18      46    T3    630      84.2      100.3  19      57          660      81.7      97.3   20      58          690      79.3      94.4   21      60          720      76.4      91.2   22      63    T4    630      82.5      98.1   18      50          660      80.7      96.2   18      53          690      79.2      93.8   19      57          720      76.7      91.0   20      58    T5    630      84.0      100.1  19      55          660      82.1      97.6   20      58          690      80.2      94.9   20      56          720      76.2      90.7   21      60    ______________________________________

                  TABLE 9    ______________________________________    Results of Tensile Test ( B-series )          Tempering                   0.2% Proof                             Tensile                                    Rupture Reduction    Steel Temp.    Stress    Strength                                    Elongation                                            of Area    No.   (°C.)                   (kgf/mm.sup.2)                             (kgf/mm.sup.2)                                    ( % )   ( % )    ______________________________________    B0    670      82.8      94.7   19      70          700      79.7      91.7   21      73          730      70.6      82.3   20      71          760      54.5      69.6   23      75          780      49.3      66.1   26      76          800      46.2      63.2   27      77    B1    670      83.0      95.3   19      72          700      80.1      92.3   21      72          730      75.3      87.7   21      73          760      61.1      74.7   23      72          780      52.9      68.9   25      77          800      47.3      64.9   28      77    B2    670      84.2      96.9   19      72          700      81.2      93.9   20      72          730      75.8      88.5   20      72          760      63.2      76.8   22      73          780      55.0      70.7   25      75          800      48.4      66.1   27      77    B3    670      82.8      96.9   20      70          700      80.6      94.3   19      70          730      77.2      91.1   21      70          760      68.4      82.6   21      68          780      59.7      75.8   22      73          800      52.8      69.8   25      73    B4    670      84.6      99.0   10      70          700      82.2      96.0   20      72          730      77.6      92.1   21      72          760      70.2      84.1   20      72          780      60.7      76.3   23      74          800      54.0      71.4   25      75    B5    670      86.4      100.3  20      70          700      83.5      97.1   20      70          730      78.4      91.5   21      68          760      62.0      82.1   20      72          780      57.1      74.1   23      73          800      52.3      70.5   27      73    ______________________________________

                  TABLE 10    ______________________________________    Results of Tensile Test at Room Temperature (T, B-series)            0.2% Proof                      Tensile    Rupture                                        Reduction    Steel   Stress    Strength   Elongation                                        of Area    No.     (kgf/mm.sup.2)                      (kgf/mm.sup.2)                                 ( % )  ( % )    ______________________________________    T0      77.5      91.4       21     59    T1      83.0      100.4      18     47    T2      81.5      99.2       17     48    T3      78.8      94.1       21     60    T4      78.7      93.4       20     57    T5      80.6      95.6       20     58    B0      57.8      72.1       22     77    B1      62.9      77.2       22     74    B2      63.5      78.1       24     74    B3      70.2      84.0       20     70    B4      72.0      86.1       19     73    B5      73.5      86.8       19     70    ______________________________________

                  TABLE 11    ______________________________________    Results of Tensile Test at 600° C. (T, B-series)            0.2% Proof                      Tensile    Rupture                                        Reduction    Steel   Stress    Strength   Elongation                                        of Area    No.     (kgf/mm.sup.2)                      (kgf/mm.sup.2)                                 ( % )  ( % )    ______________________________________    T0      45.0      53.0       26     87    T1      53.3      61.1       17     66    T2      51.5      59.9       16     63    T3      52.7      58.6       20     81    T4      49.7      58.4       20     80    T5      51.1      59.2       19     79    B0      32.7      39.8       25     85    B1      34.8      42.8       27     85    B2      35.0      42.9       33     85    B3      37.8      46.2       29     85    B4      38.5      46.9       26     84    B5      41.1      48.8       22     83    ______________________________________

                  TABLE 12    ______________________________________    Results of Impact Test (T-series)                Ductile-Brittle Transition    Steel No.   Temperature (FATT)    ______________________________________    T0          14-34° C.    T1          50-60° C.    T2          53-59° C.    T3          79-90° C.    T4          88-98° C.    T5          88-99° C.    ______________________________________

                  TABLE 13    ______________________________________    Results of Impact Test (B-series)    Steel No.   Absorbed Energy at 0° C.    ______________________________________    B0          17.5 kgf · m    B1          17.8 kgf · m    B2          17.1 kgf · m    B3          10.4 kgf · m    B4          11.8 kgf · m    B5          11.0 kgf · m    ______________________________________

                  TABLE 14    ______________________________________    Results of Creep Rupture Test (T-series)          Testing  Testing   Testing                                    Ruputure                                            Reduction    Steel Temp.    Stress    Time   Elongation                                            of Area    No.   (°C.)                   (kgf/mm.sup.2)                             (hr)   (%)     (%)    ______________________________________    T0    650      24.5       23.5  23      86    T1    650      24.5      305.3  19      60    T2    650      24.5      192.2  23      72    T3    650      24.5      459.4  16      68    T4    650      24.5      284.5  22      79    T5    650      24.5      578.3  18      58    ______________________________________

                  TABLE 15    ______________________________________    Results of Creep Rupture Test (T-series)          Testing  Testing  Testing Ruputure                                           Reduction    Steel Temp.    Stress   Time    Elongation                                           of Area    No.   (°C.)                   (kgf/mm.sup.2)                            (hr)    (%)    (%)    ______________________________________    B0    650      15.5     250.0   25     77    B1    650      15.5     1233.1  20     73    B2    650      15.5     1343.6  19     72    B3    650      15.5     1205.6  22     79    B4    650      15.5     1594.7  22     78    B5    650      15.5     (2277.3)*    ______________________________________     Note:     *; under testing

                  TABLE 16    ______________________________________    Creep Rupture Strength (T-series)            10.sup.5  hr Creep Rupture Strength (kgf/mm.sup.2)    Steel No. 580° C.                      600° C.                                 625° C.                                       650° C.    ______________________________________    T0        21.8    17.2       12.3   8.4    T1        28.9    23.9       17.8  11.7    T2        28.1    22.8       17.0  10.6    T3        29.8    25.0       19.1  13.1    T4        28.5    23.8       18.5  12.9    T5        30.1    25.5       20.0  14.7    ______________________________________

                  TABLE 17    ______________________________________    Creep Rupture Strength (B-series)             10.sup.5  hr Creep Rupture Strength (kgf/mm.sup.2)    Steel No.  600° C.                            625° C.    ______________________________________    B0         14.9         11.3    B1         17.2         13.3    B2         17.5         13.4    B3         18.4         14.2    B4         18.6         14.4    B5         21.9         15.1    ______________________________________

                  TABLE 18    ______________________________________    Results of Maximum Hardness Measurement    on Heat Affected Zone (Hv 10)            Maximum        Hardness    Steel   Hardness       of Parent                                    ΔHv    No.     of HAZ (A)     Metal (B)                                    (A-B)    ______________________________________    B0      402            227      175    B1      411            252      159    B2      417            260      157    B3      417            273      144    B4      422            274      148    B5      405            277      128    ______________________________________

What is claimed is:
 1. A method of producing a ferritic heat resistantsteel substantially free of delta-ferrite and having a body centeredcubic crystal structure and containing alloying elements whereind-electron orbital energy levels (Md) of the alloying elements and bondorders (Bo) of the alloying elements relative to iron (Fe) aredetermined by a Dv-Xα cluster method, the method comprising stepsof:selecting individual alloying elements and amounts thereof such thatan average Bo value which is expressed by {average Bo value=ΣXi·-(Bo)i}is in range of 1.805 to 1.817, and an average Md value which isexpressed by {average Md value=ΣXi·(Md)i} is in range of 0.8520 to0.8628, wherein Xi is atomic fraction of an alloying element i, and(Bo)i and (Md)i are Bo value and Md value for the alloying element i,respectively, wherein the Md values are 2.497 for Ti, 1.610 for V, 1.059for Cr, 0.0854 for Mn, 0.825 for Fe, 0.755 for Co, 0.661 for Ni, 0.637for Cu, 3.074 for Zr, 2.335 for Nb, 1.663 for Mo, 3.159 for Hf, 2.486for Ta, 1.836 for W, 1.294 for Re, -0.230 for C, -0.400 for N and 1.034for Si and the Bo values are 2.325 for Ti, 2.268 for V, 2.231 for Cr,1.902 for Mn, 1.761 for Fe, 1.668 for Co, 1.551 for Ni, 1.361 for Cu,2.511 for Zr, 2.523 for Nb, 2.451 for Mo, 2.577 for Hf, 2.570 for Ta,2.512 for W, 2.094 for Re, 0 for C, 0 for N and 0 for Si, and preparingthe substantially delta ferrite-free ferritic heat resistant steelcontaining the elements and amounts thereof in the selected step.
 2. Amethod of producing the ferritic heat resistant steel according to claim1, wherein the preparing step comprises melting the steel, the steelconsisting essentially of, in weight %, 9-13.5% Cr, 0.02-0.14% C,0.5-4.3% Co and 0.5-2.6% W, balance Fe and incidental impurities.
 3. Amethod of producing the ferritic heat resistant steel according to claim1, wherein the preparing step comprises melting the steel, the steelconsisting essentially of, in weight %, 0.07-0.14% Cr, 0.01-0.10% N,≦0.10% Si, 0.12-0.22% V, 10.0-13.5% Cr, ≦0.45% Mn, 0.5-4.3% Co,0.02-0.10% Nb, 0.02-0.8% Mo, 0.5-2.6% W, ≦0.02% B, ≦3.0% Re, ≦0.40% Ni,balance Fe and incidental impurities.
 4. A method of producing theferritic heat resistant steel according to claim 1, wherein thepreparing step comprises melting the steel, the steel consistingessentially of, in weight %, 0.02-0.12% C, 0.01-0.10% N, ≦0.50% Si,0.15-0.25% V, 9.0-13.5% Cr, ≦0.45% Mn, 0.5-4.3% Co, 0.02-0.10% Nb,0.02-0.8% Mo, 0.5-2.6% W, ≦0.02% B, ≦3.0% Re, ≦0.40% Ni, balance Fe andincidental impurities.
 5. A method of producing the ferritic heatresistant steel according to claim 1, wherein the preparing stepcomprises melting the steel consisting essentially of, the steel, inweight %, 0.07-0.14% C, 0.01-0.10% N, ≦1.10% Si, 0.12-0.22% V,10.0-13.5% Cr, ≦0.45% Mn, 0.5-4.3% Co, 0.02-0.10% Nb, 0.02-0.8% Mo,0.5-2.6% W, 0.001-0.02% B, ≦3.0% Re, ≦0.40% Ni, balance Fe andincidental impurities.
 6. A method of producing the ferritic heatresistant steel according to claim 1, wherein the preparing stepcomprises melting the steel, the steel consisting essentially of, inweight %, 0.02-0.12% C, 0.01-0.10% N, ≦0.50% Si, 0.15-0.25% V, 9.0-13.5%Cr, ≦0.45% Mn, 0.5-4.3% Co, 0.02-0.10% Nb, 0.02-0.8% Mo, 0.5-2.6% W,≦0.001-0.02% B, ≦3.0% Re, ≦0.40% Ni, balance Fe and incidentalimpurities.
 7. A method of producing the ferritic heat resistant steelaccording to claim 1, further comprising forming the steel into astructural member of a turbine.
 8. A method of producing the ferriticheat resistant steel according to claim 1, further comprising formingthe steel into a structural member of a boiler.